SOME AQUATIC INSECTS 73 



rectum a volume of water from which the air is 

 extracted by the fine tracheal branches lining 

 the rectum ; while later on, towards the close of 

 the larval stage, the insect may, and in several 

 species does, rise to the surface of the water and 

 draw in through the thoracic spiracles a supply 

 of air direct from the atmosphere. The vitiated 

 water that has been drawn into the rectum may 

 be either expelled gently and slowly when the 

 insect is at rest, or suddenly and with consider- 

 able force should it desire to move quickly 

 forward. Indeed, this sudden expulsion of water 

 is often employed by the Dragon-fly larva to 

 project itself rapidly forward so as to escape 

 some approaching foe. There is no dormant 

 stage in the life-history of the Dragon-fly that is 

 equivalent to the pupa of a Butterfly, the so- 

 called nymph only differing from the larva in 

 the appearance on its back of the wing-cases 

 containing the developing wings of the perfect 

 insect ; in all other respects it leads exactly the 

 same life as the larva, sucking the blood of all 

 unwary creatures that come within its reach, 

 and moving slowly and cautiously amongst the 

 submerged stems of the water plants. 



The most extraordinary anatomical detail of 

 the Dragon-fly larvae and so-called nymphs is the 

 complex character of their lower " lip," which 

 the insects employ in the capture of their prey. 

 The Dragon-fly larva does not chase its prey, 

 but lies in wait until its victim comes within 

 reach, often remaining almost motionless for 

 hours, clinging by its legs to the stem or branches 

 of some water weed, until some unsuspecting 



